Laser-beam carburizing of low-alloy steels

The present study investigated the use of a laser-beam, in order to carburi ze the surface of DIN 15CrNi6 low-alloy, case-hardening steel. The surface of the material was coated with graphite prior to laser irradiation. Two di fferent kinds of coatings were used: (i) a dilute commercial graphite spray ; and (ii) a slurry of graphite powder in ethanol. A CO, laser-beam was use d as the heat source, in order to activate the introduction of carbon in th e steel surface. Carburizing was achieved by two distinct mechanisms: (a) t he surface alloying mechanism, which incorporates melting of the substrate and dissolution of the graphite in the liquid phase; and (b) the solid-stat e diffusion mechanism, which incorporates austenitization of the substrate and carbon diffusion in austenite. A variety of microstructures and microha rdness profiles were produced, depending mainly on the type of graphite coa ting used, as well as on processing parameters. In general, the carburized layer was accompanied by a heat-affected zone, which was also significantly hardened, mainly due to secondary hardening. Cracking and porosity was obs erved in some specimens carburized by the surface alloying mechanism, as a result of carbon enrichment and high solidification rates. Finally, an effo rt was made to investigate the possibility for solid-state diffusion of car bon in austenite, under the short heating times imposed by laser treatment, with the use of computational kinetics simulation. (C) 2001 Elsevier Scien ce B.V. All rights reserved.